Face milling cutters are commonly used to mill a flat gasket surface on a workpiece, such as the joint face surface of a cast iron engine cylinder head. Such surfaces must be smooth, but not so smooth as to cause the gasket to slip. Just as important is the waviness of the surface. "Waviness" is basically a mathematical measure of the disparity of peaks and valleys in the surface, which, if too severe, can prevent a gasket from compressing evenly, and threaten seal integrity.
Face milling cutters typically have a cylindrical cutter body with a circular array of cutting inserts mounted on the outer edge. These are sometimes called roughing inserts, because they take the initial cut. Such inserts are often rectangular or square, a plurality of cutting corners and edges. Generally, a roughing insert is mounted in a pocket cut into the outer edge of the cutter body, and is tipped in toward the axis of the cutter body with what is known as a lead angle. A lead angle assures that only the corner, and a short length of insert edge inboard of the corner, touches the milled surface. This provides clearance between the corner from the milled surface.
Because of the short, discrete length of the part of the roughing inserts that actually touches the workpiece milled surface, they inevitably leave behind visible machining marks, sometimes called scallops. This may cause an unacceptable level of waviness in the milled surface, if the cutter is advanced at too rapid a feed rate. Lowering the feed rate can decrease waviness, but productivity is directly proportional to feed rate. Therefore, so called finishing or "wiper" inserts are often mounted radially inboard of the roughing inserts with a very slight axial step, just enough to finish up and smooth the surface. Wiper inserts have long cutting edges, and are mounted with essentially no lead angle, so that most of those long cutting edges see the surface. This reduces waviness, but can actually leave a surface that is too smooth, as evidenced by a distinct surface shine. A too smooth surface also threatens seal integrity, since the seal material can slip on the surface.